Without limiting the scope of the present invention, its background will be described with reference to perforating a hydrocarbon bearing subterranean formation with a shaped-charge perforating apparatus, as an example.
After drilling the section of a subterranean wellbore that traverses a hydrocarbon bearing subterranean formation, individual lengths of metal tubulars are typically secured together to form a casing string that is positioned within the wellbore. This casing string increases the integrity of the wellbore and provides a path through which fluids from the formation may be produced to the surface. Conventionally, the casing string is cemented within the wellbore. To produce fluids into the casing string, hydraulic openings or perforations must be made through the casing string, the cement and a distance into the formation.
Typically, the perforations are created by detonating a series of shaped-charges located within one or more perforating guns that are deployed within the casing string to a position adjacent the desired formation. A firing head assembly is deployed in the work string housing the perforation guns to initiate detonation of the shaped charges. Several techniques have been used to actuate perforating guns, including electrically, through drop-bar mechanisms, and through pressure-actuated mechanisms. A common type of firing head for detonating the perforation guns, is a differential pressure firing head; that is, a firing head which is activated by a pressure differential applied across the firing head.
One commonly used technique for conveying the perforating guns and associated apparatus into the well is to assemble the same on a tubing string, thus providing what is commonly referred to as a tubing conveyed perforating system. Such tubing conveyed perforating systems are available from the Halliburton Reservoir Services division of Halliburton Company, the assignee of the present invention. Perforating guns and associated apparatus can also be deployed on a wireline or coiled tubing.
One commonly used operating system for tubing conveyed perforating systems is a firing head which operates in response to a pressure differential. The pressure differential is typically created by applying increased pressure, either to the tubing string or to the annulus surrounding the tubing string, and conveying the increased pressure to one side (the high pressure side) of an actuating piston contained in the firing head. Typically, such a firing head will have hydrostatic pressure balanced across the actuating piston as the tool is run into the well. When it is desired to operate the tool, increased pressure is applied to the high pressure side of the actuating piston. Some prior art designs have created a pressure differential by increasing tubing pressure, above hydrostatic pressure, on the high pressure side of the piston, where the low pressure reference is hydrostatic pressure. Similarly, some firing head apparatus are actuated by maintaining tubing pressure while reducing hydrostatic pressure, thus creating a pressure differential across the firing head where the hydrostatic pressure is the low pressure. Another approach utilizes an isolated low pressure chamber (often, atmospheric) positioned within or adjacent a firing head as a low pressure reference zone. The firing head actuates in response to increased tubing pressure which creates a pressure differential compared to the low pressure chamber which is in constant communication with the low pressure side of the actuating piston. Other methods employ a low pressure chamber (e.g., atmospheric) positioned in a fluid chamber which is initially open to hydrostatic or tubing pressure. The system is pressure balanced until the low pressure chamber is opened, at which point the fluid pressure in the fluid chamber drops, creating a low pressure reference for firing the head.
Disclosure regarding methods for actuating firing heads and types of differential firing heads can be found in the following references, which are each incorporated herein by reference for all purposes: U.S. Pat. No. 5,301,755, to George; U.S. Pat. No. 4,917,189, to George; U.S. Pat. No. 5,161,616, to Colla; U.S. Pat. No. 4,566,544 to Bagley; U.S. Pat. No. 4,616,718 to Gambertoglio; and U.S. Pat. No. 5,297,718 to Barrington.
There are disadvantages to using firing heads which require substantial pressure to be applied to the tubing or annulus to provide the increase in pressure which actuates the tool. In some instances, the pressures necessary to actuate the tools may be excessively high. Also, in many well perforation jobs it is desirable to perforate in an underbalanced condition, that is with a relatively low pressure present in the well annulus when perforating occurs, and thus if high pressures are applied to actuate the perforating gun, it is necessary to be able to bleed off those high pressures very rapidly before the well is actually perforated. Further, in some situations it is preferable to actuate the perforating guns with no applied tubing or hydrostatic pressure. Thus it is seen that there is a need for a pressure actuated firing system which can avoid or eliminate the application of excessively high pressures.